Tacx Neo vs. Stages power meter 20 watt difference?

I’m new and have searched the forums. I see a lot of discussion about calibrating power meters. I have a Tacx Neo. I’m just beginning training and did an FTP test that gave me 170 watts. I’ve since added a Stages PM and when I compare wattage between the the Stages is consistently 20ish watts lower than the Neo.

I’ve calibrated the Stages, but the Tacx Utility app won’t seem to prompt mt to calibrate the Neo. It connects and reports power but won’t prompt me to calibrate it. I’ve read you can’t calibrate the Neo, though Tacx has a blog entry about doing so on direct drive units. That may not apply to Tacx. I don’t know.

So, which should I trust? When doing my workouts it currently picks up the Stages, and it’s not very challenging.

Any ideas?

You can’t calibrate a Neo. Which Neo BTW? Neo, Neo 2, Neo 2T?

Left side only Stages or double sided Stages? What gearing are you using when you tested this? I think you’ll find the variance changes by gearing, it may also vary by wattage you’re hitting. Though, for me, I don’t really get separation until I’m around 250W on one power meter, and more separation against another power meter when I change to the small ring and different gears in back. Which is right, I have no idea. I guess I can put another bike on, with a different power meter on and compare again.

Trainer power seems to be a crapshoot. I think power meters are probably similar in that vein, unless you have multiples to check against and can get within a range.

It’s a NEO v1 and a left sided Stages. The variance seems to be pretty constant. One thing I noticed when running the app with the Stages PM is that on a couple of occasions the power output seemed to spike and it was difficult to bring the power into a reasonable range.

This morning I did a workout that included intervals that were supposed to be about 144 watts. It asked me to get out of the saddle and spin for a while at 60is RPM. I basically couldn’t do it without spinning out. It seemed like the stages was just really high and so even when I was supposed to be working hard it was easy. I’d love for that to be the case but I don’t believe it.

Poking around I suspect it’s a variety of issues popping up. Natural accuracy differences plus drive train losses.

Since I’m doing some training outdoors, maybe the thing to do is another FTP test on the trainer to get a new baseline. My guess is it would be 190watts. which would get me much closer to where I want to be.

You could have a left side bias, which would inflate your numbers as well. I use PMs that are pretty close to the Neo 2T I have. I have one that is a bit farther off, the PowerTap C1 that is on my trainer bike (permanently mounted on the trainer) but I also have the Assioma DUO on those. For the record, my imbalance seems to be 46%L, 54%R on average, even dipping to 40/60 at times. So, if I had a Stages L only, my FTP would read lower, and those numbers would vary quite a bit as I move around on an outdoor bike way more than I do on a trainer bike.

It’s one reason I never like a L only, that only sees half the picture and then doubles it.

But, here’s some info for you as well, as it pertains to gearing.

https://tacxfaqx.com/knowledge-base/linearity-showdown/

Certainly possible. Part of the reason I’ve gotten on the bike is that I’ve developed a bad case of Patellar tendonitis on the right and I can’t really run these days. So it’s certainly possible I’m left biased.

I think I’ll just re-baseline with the Stages and call it good.

Depending on the gear you’re using when you’re on the trainer you should expect 8W to 12W loss from a stages left crank power meter to a rear wheel power measurement. If you’re chain isn’t clean and/or you need to replace your jockey wheels it could be worse than that, I guess.

I once helped a guy set up his three-wheeled bike on a trainer. The difference in power read from an orthogonal array of strain gauges on the crank vs the power read on an optical torque sensor at the rear hub was consistently 24W to 26W. So it can happen.

Test the Neo against a SRM or a third PM if you can and hopefully two of three will agree (unless it’s a SRM which is the gold standard). My money is on the Stages reading high. My Stages was about 20w off compared to my Pioneer dual and the Pioneer was indicting 46/54 split. The same split is also reported by my Assioma Duo. I never tested my Stages (sold it) against my trainer, Elite Drivo, but my Pioneer and Assioma tracked very closely it.

I missed this part. This may be a communication error. But it should recover within about 5-10 seconds.

That’s not a PM problem. I can never do this. 144W is insanely low, and to do that out of the saddle means I need to hold myself up on the bars and spin my legs. Out of the saddle, for me, is a minimum 200+W effort. Anything less is not worth the time and extra strain to prop myself up on the bars and spin my legs.

I’ve had that on my left side. In fact, it seems most of my problems happen on the left side.

That’s about all you can do.

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@Erik_Wahlstrom here are the drive train losses attributable to a system with new chainset, new cassette, cleaned chain lubed with additive free mineral oil. Just for your reference. If you’re stages power meter is NOT a little bit higher than your trainer power reading then you should be suspicious.

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Sorry I don’t quite understand. How would I know if I’m running in Erg? It is quite noisy with a low grinding sort of sound. How would I pop up a shot?

@Erik_Wahlstrom
What length crank arm do you have?
There should be an option in the Stages app to change the crank arm length. In my case, adjusting the crank length to a shorter setting has allowed me to get the StagesL G3 on my MTB pretty close to my Neo2. I’ve done much the same thing with my 4iiii on the road bike using the scale factor setting.
To test the results I made dual recordings of the ramp test with sprints thrown in during the cool down. The power doesn’t align perfectly of course, but it’s allowed me to get things pretty close from 150 -800w which is good enough for me. YMMV.

It’s been great since I did this as I can jump on any bike and not have to worry about making adjustments on the fly depending on the power source. Having numbers that line up between different sources is the goal in my case.

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Ok, here’s my ramp test using the stages meter.

FTP came out at 216, which seemed quite high. Using a Tacx test (very different) they gave me a 170.

I’ve got no idea which is more accurate. Any thoughts? I’d love the 216 number to be real. But . . .

Doing a factory reset on my Stages really helped. It was reading low.

After my experience with my Tacx Vortex, I’d usually trust a Stages over a trainer.

I have a left Stages. I think it is pretty consistent. My Kickr reads about 5-10 watts higher. My powertap hub reads 5 watts low.

I started using the Stages for all measurements, indoors and out. One power meter is the best policy IMO.

Before you try and analyse the problem any further, you need to do a dual recording. Do a free ride session (ie not in ERG mode), recording the Neo and Stages simultaneously to separated sources - ie Neo to trainerroad, Stages to your Garmin. Upload both and see what the difference shows. You can use the analysis tool on Zwiftpower for this, or the DCR analysis tool.

Then lets see how the power compares. If I was to place a bet, I’d suspect your Neo is going to giveyou a truer value than the stages.

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40+ Watts difference. Over 20%. I think something is screwed up somewhere, for sure!

Doesn’t TACX have a diagnostic dashboard in their trainer app? Does everything look ok there?

Do you get a different result when out if the saddle? If the big discrepancy goes away when you are standing on the pedals then that might point point to L/R imbalance being the issue.

Thanks all. I’ve decided to just uses the stages meter. After more reading and riding I think I can chalk up the differences to

  1. Drivetrain losses. When I did the test I’d not cleaned my chain in . . . . Well ever. Degreasing and lube loosened it up.
  2. Leg imbalance. My right is problematic due to injury
  3. Bad testing. I was winging it on the neo.

Since I plan to ride outdoors a lot with power I’ll just stick to the stages ftp meter. I did the Reinstein ride outside with the 216 FTP and it felt suitably hard but not insane.

I encountered the same issue. The Neo 2T gives in average a 20w lower power reading than the stages ( left only). I also have an Assomia duo which gives same reading as the stages. Furthermore when I do a theoretical calculation ( Wolfgang-Menn.de, recommended by GCN) using data from various steep climbs I did on my road-bike the calculated power is very close to my readings on stages/assomia. As those calculations are based on basic physics my conclusion is that the Neo 2t ( at least mine) under estimates power output. If you want to get to the same number you can just play with your weight. In any case the absolute numbers are not that important the importance is the consistency of the measurement so you shouldn’t get too obsessed by the number. Once you know one measures always 20w less you just adapt your settings to that so on my road bike my ftp is 260 and on the Tacx it’s 240

I may have a post way way up there, but exactly the same on my Neo-original flavor. Same deviation 20W or so, deviating higher with higher power outputs and it is not cadence dependent.

Interesting, the 20w difference is quite stable in my case when looking at the average of a training at different watt levels but max power during a session is very similar